Method of producing metal-coated steel strip

ABSTRACT

A method of forming a coating of an Al—Zn—Si—Mg alloy on a steel strip to form an Al—Zn—Mg—Si coated steel strip is disclosed. The method includes the steps of dipping steel strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on exposed surfaces of the steel strip and cooling the coated strip with cooling water. The cooling step includes controlling the pH of cooling water to be in a range of p H 5-9. Particular embodiments focus on Al—Zn—Si—Mg alloys that contain the following elements in % by weight: Zn: 30 to 60, Si: 0.3 to 3, Mg: 0.3 to 10, and Balance Al and unavoidable impurities.

TECHNICAL FIELD

The present invention relates to the production of metal strip,typically steel strip, which has a coating of a corrosion-resistantmetal alloy that contains aluminium-zinc-silicon-magnesium as the mainelements in the alloy, and is hereinafter referred to as an “Al—Zn—Si—Mgalloy” on this basis.

In particular, the present invention relates to a hot-dip metal coatingmethod of forming a coating of an Al—Zn—Si—Mg alloy on a strip thatincludes dipping uncoated strip into a bath of molten Al—Zn—Si—Mg alloyand forming a coating of the alloy on the strip.

Typically, the Al—Zn—Si—Mg alloy of the present invention comprises thefollowing ranges in % by weight of the elements Al, Zn, Si, and Mg:

-   -   Zn: 30 to 60%    -   Si: 0.3 to 3%    -   Mg: 0.3 to 10%    -   Balance Al and unavoidable impurities.

More typically, the Al—Zn—Si—Mg alloy of the present invention comprisesthe following ranges in % by weight of the elements Al, Zn, Si, and Mg:

-   -   Zn: 35 to 50%    -   Si: 1.2 to 2.5%    -   Mg 1.0 to 3.0%    -   Balance Al and unavoidable impurities.

The Al—Zn—Si—Mg alloy may contain other elements that are present in thealloy as deliberate alloying additions or as unavoidable impurities.Hence, the phrase “Al—Zn—Si—Mg alloy” is understood herein to coveralloys that contain such other elements as deliberate alloying additionsor as unavoidable impurities. The other elements may include by way ofexample any one or more of Fe, Sr, Cr, and V.

It is noted that the composition of the as-solidified coating of theAl—Zn—Si—Mg alloy may be different to an extent to the composition ofthe Al—Zn—Si—Mg alloy used to form the coating due to factors such aspartixdla dissolution of the metal strip into the coating during thecoating process.

Depending on the end-use application, the metal-coated strip may bepainted, for example with a polymeric paint, on one or both surfaces ofthe strip. In this regard, the metal-coated strip may be sold as an endproduct itself or may have a paint coating applied to one or bothsurfaces and be sold as a painted end product.

BACKGROUND ART

One corrosion resistant metal alloy coating that is used widely inAustralia and elsewhere for building products, particularly profiledwall and roofing sheets, is an Al—Zn alloy coating, more particularly acoating formed from a 55% Al—Zn alloy that also comprises Si in thealloy. The profiled sheets are usually manufactured by cold formingpainted, metal alloy coated strip. Typically, the profiled sheets aremanufactured by roll-forming the painted strip.

The addition of Mg to this known 55% Al—Zn alloy has been proposed inthe patent literature for a number of years, see for example U.S. Pat.No. 6,635,359 in the name of Nippon Steel Corporation.

It has been established that when Mg is included in a 55% Al—Zn alloycoating, Mg brings about certain beneficial effects on productperformance, such as improved cut-edge protection.

The applicant has carried out extensive research and development work inrelation to Al—Zn—Si—Mg alloy coatings on strip such as steel strip. Thepresent invention is the result of part of this research and developmentwork.

The above discussion is not to be taken as an admission of the commongeneral knowledge in Australia and elsewhere.

SUMMARY OF THE INVENTION

The research and development work that is relevant to the presentinvention included a series of plant trials on metal coating lines ofthe applicant to investigate the viability of forming Al—Zn—Si—Mg alloycoatings on steel strip on these metal coating lines. The plant trialsfound that Al—Zn—Si—Mg alloy coatings are far more reactive with quenchwater used to cool metal alloy coatings on strip after coated stripleaves molten alloy baths in the metal coating lines than conventionalAl—Zn coatings. More particularly, the applicant found that there wasgreater dissolution of Al—Zn—Si—Mg alloy coatings into quench water thanwas the case with conventional Al—Zn coatings and the dissolutionresulted in precipitates in quench water that caused a rapiddeterioration of cooling water circuit heat exchangers and causedundesirable coatings to form on cooling water storage tank surfaces inthe quench water circuits in the metal coating lines. The precipitationproblem is a potentially serious maintenance issue.

After identifying the precipitate problem and carrying out furtherresearch and development work, the applicant found that pH control ofcooling water and to a lesser extent cooling water temperature controlmade it possible to reduce the extent of precipitate formation andallowed the cooling water heat exchangers to perform in a practicalmanner. More particularly, the applicant found that the precipitateproblem could be addressed by suppressing the alkalinity of coolingwater via pH control of cooling water and to a lesser extent coolingwater temperature control (operating at low temperatures) to therebyreduce the corrosiveness of the cooling water towards Al—Zn—Si—Mg alloycoatings.

According to the present invention there is provided a method of forminga coating of an Al—Zn—Si—Mg alloy on a steel strip to form anAl—Zn—Mg—Si coated steel strip, the method including the steps ofdipping steel strip into a bath of molten Al—Zn—Si—Mg alloy and forminga coating of the alloy on exposed surfaces of the steel strip andcooling the coated strip with cooling water, with the cooling stepincluding controlling the pH of cooling water to be in a range of pH5-9.

The cooling step may include controlling the pH of cooling water to beless than 8.

The cooling step may include controlling the pH of cooling water to beless than 7.

The cooling step may include controlling the pH of cooling water to beless than 7.5.

The cooling step may include controlling the pH of cooling water to begreater than 5.5.

The cooling step may include controlling the pH of cooling water to begreater than 6.

The cooling step may include controlling the temperature of coolingwater to be in a range of 25-80° C.

The cooling step may include controlling the temperature of coolingwater to be less than 70° C.

The cooling step may include controlling cooling water temperature to beless than 60° C.

The cooling step may include controlling cooling water temperature to beless than 55° C.

The cooling step may include controlling cooling water temperature to beless than 50° C.

The cooling step may include controlling cooling water temperature to beless than 45° C.

The cooling step may include controlling cooling water temperature to begreater than 30° C.

The cooling step may include controlling cooling water temperature to begreater than 35° C.

The cooling step may include controlling cooling water temperature to begreater than 40° C.

The cooling step may include controlling the pH by adding acid to thecooling water.

The cooling step may include controlling the pH by adding acid and othersalts, buffers, wetting agents, surfactants, coupling agents, etc.

The acid may be any suitable acid such as phosphoric acid and nitricacid by way of example.

The cooling step may be a water quench step.

The cooling step may be a closed loop in which water is circulatedthrough a circuit that supplies water to the coated strip and collectsand cools water and returns the cooled water for cooling the coatedstrip.

The closed loop may include a water storage tank, a spray system forsupplying water to the coated strip from the tank, and a heat exchangerfor cooling water after it has been sprayed onto the strip.

The cooling step may be an open loop in which cooling water is notrecycled in the cooling step.

The cooling step may include controlling the operating conditions tocool the coated strip to a temperature range of 28-55° C.

The cooling step may include controlling the operating conditions tocool the coated strip to a temperature range of 30-50° C.

The method may include other steps including any one or more of thesteps of pre-treating strip to clean the strip before the hot dipcoating step, controlling the thickness of the coated strip immediatelyafter the coating step, rolling the coated strip, treating the coatedstrip with a passivation solution, and coiling the coated strip.

The Al—Zn—Si—Mg alloy may include more than 0.3% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 1.0% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 1.3% by weight Mg.

The Al—Zn—Si—Mg alloy may comprise more than 1.5% by weight Mg.

The Al—Zn—Si—Mg alloy may include less than 3% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 2.5% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 1.2% by weight Si.

The Al—Zn—Si—Mg alloy may include less than 2.5% by weight Si.

The Al—Zn—Si—Mg alloy may include the following ranges in % by weight ofthe elements Al, Zn, Si, and Mg:

-   -   Zn: 30 to 60%    -   Si: 0.3 to 3%    -   Mg: 0.3 to 10%    -   Balance Al and unavoidable impurities.

The Al—Zn—Si—Mg alloy may include the following ranges in % by weight ofthe elements Al, Zn, Si, and Mg:

-   -   Zn: 35 to 50%    -   Si: 1.2 to 2.5%    -   Mg 1.0 to 3.0%    -   Balance Al and unavoidable impurities.

The Al—Zn—Si—Mg alloy coating may contain other elements that arepresent as deliberate alloying additions or as unavoidable impurities.The other elements may include by way of example any one or more of Fe,Sr, Cr, and V.

By way of particular example, the other elements may include Ca fordross control in molten coating baths.

The steel may be a low carbon steel.

The present invention also provides an Al—Zn—Mg—Si alloy coated steelstrip produced by the above-described method.

The Al—Zn—Si—Mg alloy used to form the coating of the Al—Zn—Mg—Si alloycoated steel strip may include the following ranges in % by weight ofthe elements Al, Zn, Si, and Mg:

-   -   Zn: 30 to 60%    -   Si: 0.3 to 3%    -   Mg: 0.3 to 10%    -   Balance Al and unavoidable impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described further by way of example withreference to the accompanying drawings of which:

FIG. 1 is a schematic drawing of one embodiment of a continuous metalcoating line for forming an Al—Zn—Si—Mg alloy coating on steel strip inaccordance with the method of the present invention;

FIG. 2 is a graph of the Al and Ca concentrations in cooling water usedduring the course of a plant trial carried out by the applicant; and

FIG. 3 is a graph of the Mg and Zn concentrations in cooling water usedduring the course of the plant trial carried out by the applicant.

DESCRIPTION OF EMBODIMENTS

With reference to FIG. 1, in use, coils of cold rolled low carbon steelstrip are uncoiled at an uncoiling station 1 and successive uncoiledlengths of strip are welded end to end by a welder 2 and form acontinuous length of strip.

The strip is then passed successively through an accumulator 3, a stripcleaning section 4 and a furnace assembly 5. The furnace assembly 5includes a preheater, a preheat reducing furnace, and a reducingfurnace.

The strip is heat treated in the furnace assembly 5 by careful controlof process variables including: (i) the temperature profile in thefurnaces, (ii) the reducing gas concentration in the furnaces, (iii) thegas flow rate through the furnaces, and (iv) strip residence time in thefurnaces (i.e. line speed).

The process variables in the furnace assembly 5 are controlled so thatthere is removal of iron oxide residues from the surface of the stripand removal of residual oils and iron fines from the surface of thestrip.

The heat treated strip is then passed via an outlet snout downwardlyinto and through a molten bath containing an Al—Zn—Si—Mg alloy held in acoating pot 6 and is coated with Al—Zn—Si—Mg alloy. Typically, theAl—Zn—Si—Mg alloy in the coating pot 6 comprises in % by weight: Zn: 30to 60%, Si: 0.3 to 3%, Mg: 0.3 to 10%, and balance Al and unavoidableimpurities. The coating pot 6 may also contain Ca for dross control inthe molten bath. The Al—Zn—Si—Mg alloy is maintained molten in thecoating pot at a selected temperature by use of heating inductors (notshown). Within the bath the strip passes around a sink roll and is takenupwardly out of the bath. The line speed is selected to provide aselected immersion time of strip in the coating bath. Both surfaces ofthe strip are coated with the Al—Zn—Si—Mg alloy as it passes through thebath.

After leaving the coating bath 6 the strip passes vertically through agas wiping station (not shown) at which its coated surfaces aresubjected to jets of wiping gas to control the thickness of the coating.

The exposed surfaces of the Al—Zn—Si—Mg alloy coating oxidise as thecoated strip moves through the gas wiping station and a native oxidelayer forms on the exposed surfaces of the coating. The native oxide isthe first oxide to form on the surface of the metal alloy coating, withits chemical make-up being intrinsically dependent on the composition ofthe metal alloy coating, including Mg oxide, Al oxide, and a smallamount of oxides of other elements of the Al—Zn—Si—Mg alloy coating.

The coated strip is then passed through a cooling section 7 and issubjected to forced cooling by means of a water quench step. The forcedcooling may include a forced air cooling step (not shown) before thewater quench step. The water quench step is, by way of example, a closedloop in which water sprayed onto coated strip is collected and thencooled for re-use to cool coated strip. The cooling section 7 includes acoated strip cooling chamber 7 a, a spray system 7 b that sprays wateronto the surface of the coated strip as it moves through the coolingchamber 7 a, a water quench tank 7 c for storing water that is collectedfrom the cooling chamber 7 b, and a heat exchanger 7 d for cooling waterfrom the water quench tank 7 c before transferring the water to thespray system 7 b.

In accordance with one embodiment of the present invention (a) the pH ofthe cooling water supplied to the spray system 7 b is controlled to bein a range of pH 5-9, typically in a range of 5-8, more typically in arange of 5.5-7.5 and (b) the temperature of the cooling water suppliedto the spray system is controlled to be in a relatively low temperaturerange of 30-50° C. Both control steps (a) and (b) minimise dissolutionof the Al—Zn—Si—Mg alloy coating on the coated strip.

The pH and temperature control may be achieved, by way of example, byusing a pH probe and a temperature sensor in an overflow tank of thewater quench tank 7 c and supplying data from the probe/sensor to a PLCand calculating required acid additions to maintain the pH atpredetermined set points for pH and the water temperature, with any acidadditions and temperature adjustments being made so that the water inthe water quench tank 7 c is controlled to the set points for pH andtemperature. This is not the only possible option for achieving pH andtemperature control.

The pH, temperature, and chemical control may also be achieved by way ofexample, by using a once through water cooling system where the quenchwater is not recirculated and the input water has pH and temperatureproperties as described above.

The cooled, coated strip is then passed through a rolling section 8 thatconditions the surface of the coated strip. This section may include oneor more of skin pass and tension leveling operations.

The conditioned strip is then passed through a passivation section 10and coated with a passivation solution to provide the strip with adegree of resistance to wet storage and early dulling.

The coated strip is thereafter coiled at a coiling station 11.

As discussed above, the applicant has conducted extensive research anddevelopment work in relation to Al—Zn—Si—Mg alloy coatings on steelstrip.

The research and development work included plant trials on metal coatingline MCL1 at the Springhill operations of the applicant. The planttrials found that white precipitates formed in the quench system of theline when the line was operating with Al—Zn—Si—Mg alloy as a coatingalloy for steel strip. Significantly, it was found that these whiteprecipitates eventually blocked the quench system heat exchanger. TheSpringhill metal coating lines are similar in general terms to the lineshown in FIG. 1 and include a closed loop quench step on each of thethree lines (MCL1, MCL2, and MCL3). Each closed loop processes arelatively small volume (approx 5000 L) of water. The cooling water iscooled by dedicated heat exchangers on each line. The white precipitatesformed on cooling system equipment surfaces and covered an initial layerof grey material. The grey layer was found to contain Al(OH)₃ andAl₂O₃.3H₂O from previous line operations using conventional Al—Znalloys. The white precipitates were found to contain Mg₄Al₂(OH)₁₄.3H₂Oand Al₂O₃.3H₂O. These magnesium/aluminium oxy/hydroxides also containedmagnesium carbonate compounds.

The plant trials carried out by the applicant comprised initial planttrials on Al—Zn—Si—Mg alloys in two groups of alloy compositions thatidentified the precipitate problem in the first instance and later moreextensive plant trials that confirmed the precipitate problem andevaluated several options to minimise the problem. Group (a) alloysinclude the following ranges in % by weight of the elements Al, Zn, Si,and Mg: Al: 2 to 19%, Si: 0.01 to 2%, Mg: 1 to 10%, and balance Zn andunavoidable impurities. Group (b) alloys include the following ranges in% by weight of the elements Al, Zn, Si, and Mg: Al: 30 to 60%, Si: 0.3to 3%, Mg: 0.3 to 10%, and balance Zn and unavoidable impurities.

The following description focuses on the later plant trials.

The later plant trials on the MCL1 line were carried out by hot dipcoating steel strip with the following alloys in coating baths: (a) aknown Al—Zn alloy (hereinafter referred to as “AZ”) and (b) anAl—Zn—Si—Mg alloy (hereinafter referred to as “AM”) having the followingcompositions, in wt. %:

-   -   AZ: 55Al—43Zn—1.5Si—0.45Fe-incidental impurities.    -   AM: 53Al—43Zn—2Mg—1.5Si—0.45Fe-incidental impurities (a        group (b) alloy).

The later plant trials on the MCL1 line are summarised below.

Quench System—No Control

The first week of the plant trials on the MLC1 line was run with the AZ(Al—Zn) alloy and produced standard Zincalume (Registered Trade Mark)coated strip. The line was run in accordance with established operatingconditions. In terms of the water cooling step on the line, the quenchwater was at a temperature of 50-60° C. upstream of the water sprays.There was no pH control of the quench water. Under these conditions thequench water became saturated with aluminium and the pH increased toaround 8.5 (at 60° C.)

As soon as Mg (and a small amount of Ca for dross control) was added tothe metal coating pot to adjust the AZ alloy composition to the AM(Al—Zn—Si—Mg) alloy coating composition the pH started to rise andeventually reached 10.0. The quench water became milky white and theinlet screens to the quench pumps became blocked with milky whiteprecipitates and had to be removed. The quench water was analysed andthe results of the analysis are presented in Table 1.

TABLE 1 Quench Tank White Precipitate - AM alloy Element wt % (XRF) wt %(ICP) Al 21.0 24.2 Mg 8.5 10.2 Ca 2.2 2.0 Zn 0.29 0.26 C 3.1 —

A typical Al—Zn scale is almost all aluminium. Consequently, the Table 1data indicates that a surface layer rich in Mg and Ca was dissolving inthe quench water. The proportion of Mg and Ca relative to Al in thequench deposits was much higher than in the metal pot. The presence andquantity of carbon in Table 1 also indicated that both Ca and Mg wereforming carbonates in the quench water. The white precipitates werefound to contain Mg₄Al₂(OH)₁₄.3H₂O and Al₂O₃.3H₂O. Thesemagnesium/aluminium oxy/hydroxides also contained magnesium carbonatecompounds.

The presence of the white precipitate in the quench water caused thequench heat exchanger to become blocked quickly. When operating withconventional Al—Zn alloy compositions, the quench heat exchanger on theline would typically last 9 months. The presence of magnesium andcalcium made a significant change to the surface characteristics of thecoated strip and increased the dissolution of the oxide layer during thewater cooling step.

The applicant considered a range of options to prevent or minimise thedissolution for the AM Al—Zn—Si—Mg alloy coating. The applicant settledon a strategy of suppressing the alkalinity of cooling water via pHcontrol of cooling water and to a lesser extent cooling watertemperature control to thereby reduce the corrosiveness of the coolingwater towards Al—Zn—Si—Mg alloy coatings. The plant trials tested twooptions, namely pH control and cooling water temperature control, asdiscussed below.

Quench System—pH Control

A trial to control quench tank pH using phosphoric acid ran for 4 days.The control system was set to allow a predetermined [OH⁻] ion value of1.0×10⁻⁶ mol/L.

Table 2 provides the values of the pH set point for different waterquench tank temperatures to maintain a set pH.

TABLE 2 Quench Tank pH requirements for constant [OH⁻] concentrationTemp pH Set (° C.) point 35 7.68 40 7.53 45 7.39 50 7.26 55 7.14 60 7.0265 6.90 70 6.80

The pH and the concentration of the dosing acid were 1.6 and 53.6 g/LH₃PO₄ respectively. During the trial the dosing acid consumption wasquite low, approximately 17 L/day, or less than about 1 L/day ofconcentrated phosphoric acid (85 wt %). Quench tank dosing provedeffective at controlling white precipitate formation and preventingquench heat exchanger blockage. Another outcome of pH dosing was thatthe pH probe did not foul.

Quench System—Low Temperature Control

At the end of the above-described pH control trial period, the set pointtemperature for the quench tank sprays was lowered 50° C. to 35° C., andpH dosing was discontinued. The quench tank was flushed with water toremove residual salts from the pH control trial. This change caused wetstrip conditions further downstream but it also showed that temperatureis an important variable for quench tank control. During the period ofthe low temperature operation (24 hours) there was no increase indifferential pressure across the quench tank heat exchanger. The quenchtank temperature was typically 15° C. higher than the spray temperature.During the low temperature trial the quench tank temperature was 48-50°C. rather than the 65-70° C. typical of normal MCL1 quenchingconditions.

After 24 hours the set point was increased to 50° C. to determinewhether temperature is a critical variable. The quench heat exchangerdifferential pressure started to increase immediately—indicating theformation of precipitates in the heat exchanger.

After 10 hours the set point was lowered to 40° C. but this seemed tohave little impact. When the quench heat exchanger differential pressurereached 110 kPa the set point was returned to 50° C. and the quench tankwas dosed with acid to bring the pH down and pH control was reactivated.Dosing was left on during the run down of the pot in the final days ofthe trial. The quench water became clear and the quench heat exchangerdifferential pressure stabilised during this time.

Quench Water Analysis

Samples of quench water were collected and analysed during the trials.The results are shown in FIGS. 2 and 3.

In FIGS. 2 and 3 the periods 1-4 represent pH control (1), lowtemperature control (35° C.) (2), quench tank set point at 50° C. (3),and quench tank set point at 40° C., respectively.

With reference to the Figures, both aluminium and calcium seem to followthe same trend (FIG. 2). Lower quench tank temperature and pH dosinglowered the level of these ions in the quench water, with the calciumlevels dropping substantially. Without control the level of Al in thequench water is considerably higher for Al—Zn—Si—Mg alloy coatings thanAl—Zn alloy coatings (typical Al—Zn concentrations in quench water are4-20 mg/L). The impact of pH control on magnesium concentration is shownin FIG. 3. It increased considerably during the 4 day test period.Increased magnesium levels are also evident for cooler quench tankconditions. Zinc levels also increased during pH control and for thecoldest quench tank trial (35° C.) but was still at low levels overall.

Conclusions

The above trials and other research and development work of theapplicant established that Al—Zn—Si—Mg alloy coated strip is far morereactive in cooling water than Al—Zn alloy coated strip and lead torapid deterioration of the quench heat exchangers and coatings of thequench tank surfaces, with the higher reactivity being due in large partto magnesium and calcium. Lower quench tank temperatures and pH controlreduced the impact of magnesium and calcium dissolution in the quenchwater and allowed the quench heat exchangers to perform in a practicalmanner.

Many modifications may be made to the present invention described abovewithout departing from the spirit and scope of the invention.

By way of example, whilst the embodiment of the metal coating line shownin FIG. 1 includes a coated strip cooling section 7 that includes watersprays, the present invention is not so limited and extends to anysuitable water cooling system, such as dunk or immersion tanks.

1. A method of forming an Al—Zn—Si—Mg alloy coating on a steel strip toform a coating of an Al—Zn—Mg—Si on a steel strip, with the Al—Zn—Si—Mgalloy including the following ranges in % by weight of the elements Al,Zn, Si, and Mg: Zn: 35 to 50% Si: 1.2 to 2.5% Mg: 1.0 to 3.0% Balance Aland unavoidable impurities, the method including the steps of dippingthe steel strip into a bath of molten Al—Zn—Si—Mg alloy and forming acoating of the alloy on exposed surfaces of the steel strip, cooling thecoated strip with cooling water, conditioning the surface of the coatedstrip in a rolling section, and passivating the surface of the coatedstrip to provide resistance to wet storage and early dulling by coatingthe strip with a passivation solution in a passivation section, with thecooling step including controlling the pH of cooling water to be in arange of pH 5-9 by adding acid to the cooling water and controlling thetemperature of cooling water to be in a range of 25-80° C., and with thecooling step including a water quench step and (a) a closed loop inwhich water is circulated through a circuit that supplies water to thecoated strip and collects and cools water and returns the cooled waterfor cooling the coated strip or (b) an open loop in which cooling wateris supplied from a cooling tower to the coated strip and collected andrecirculated through the cooling tower.
 2. The method defined in claim 1wherein the cooling step includes controlling the pH of cooling water tobe less than
 8. 3. The method defined in claim 1 wherein the coolingstep includes controlling the pH of cooling water to be less than
 7. 4.The method defined in claim 1 wherein the cooling step includescontrolling the pH of cooling water to be greater than
 6. 5. The methoddefined in claim 1 wherein the cooling step includes controlling coolingwater temperature to be greater than 30° C.
 6. The method defined inclaim 1 wherein the cooling step includes a closed loop and not an openloop.
 7. The method defined in claim 1 wherein the cooling step includesan open loop and not a closed loop.